Fix firstline offset under new s:lists format

autoload/startify.vim

@@ -191,8 +191,8 @@ function! startify#insane_in_the_membrane() abort
   let b:startify.firstline = 2
   let b:startify.firstline += len(g:startify_header)
   " no special, no local Session.vim, but a section header
-  if !s:show_special && !exists('l:show_session') && type(s:lists[0]) == type([])
-    let b:startify.firstline += len(s:lists[0]) + 1
+  if !s:show_special && !exists('l:show_session') && has_key(s:lists[0], 'header')
+    let b:startify.firstline += len(s:lists[0].header) + 1
   endif
 
   let b:startify.lastline = line('$')